Communication and power cables, which operate exposed to the environment, often develop faults between the insulated conductors or between the metallic shields or armor and earth. Locating and repairing damage to communication and power cables presents a particular challenge when certain types of fault develop. A conductor to conductor or conductor to ground fault caused by moisture is particularly difficult to locate. This type of fault tends to be erratic, with the ohmic value of the fault resistance changing with applied voltage and polarity over time. The unpredictable and electrically noisy nature of such faults makes fault locating using simple loop measurements from the ends of the faulted conductor(s) ineffective.
A measurement method and device to address this problem is described by Vokey et al U.S. Pat. No. 4,947,469. The patent details a method whereby instruments are placed at both ends of the faulted conductor(s). A series of simultaneous measurements are then carried out by carefully synchronizing the instruments such that the voltage and current readings are recorded at both ends at the same instant. The instrument at the remote end transmits its measurement data to the first instrument for computation. The conductor resistances are then calculated as follows: ##EQU1## Where: V.sub.a is the voltage at end a, l.sub.a is the current at end a;
V.sub.b is the voltage at end b, l.sub.b is the current at end b;
i is the ith simultaneous measurement; and
N is the number of simultaneous measurements.
A fundamental limitation exists in this method. The simultaneous measurement of the voltage and current at the conductor ends assumes that steady state conditions will predictably exist at both ends at the same instant.
In practical applications such as locating damage to the plastic and steel protective outer jacket on buried fiber optic communication cables, the conductor ends can be 100 km or more apart. At these distances the series resistance and parallel capacitance to ground of the protective steel armor gives rise to transmission line phase delays. Disturbances along the conductor from external electrical influences such as power lines or telluric earth currents affect the ends of the conductors differently at different times. These disturbances coupled with time delays result in different transient voltage and current conditions at different times at the conductor ends. These transient voltages can introduce measurement errors at the precise time of the simultaneous measurements.
The present invention is concerned with a determination of the distance to a conductor fault without the inherent susceptibility to errors caused by transient voltages and other electrical disturbances.